A typical situation where a position detector is used is for example as a component in a revolution counter or tachometer which outputs an electrical pulse and evaluates and stores it electronically, whenever a rotating shaft passes through a preselected angular position.
An example of a situation of use of such a revolution counter is in connection with machine tools in which a coarse measurement value in relation to the position of the tool holder carriage or slide is obtained by counting off the number of revolutions of the spindle which displaces the carriage or slide. A problem arises in that situation by virtue of the fact that, in the event of the power supply for the electronic processing system being switched off or failing, the coarse measurement value in respect of the carriage position should not be lost and should be immediately available again after the power supply is switched on again, even if, during the period for which there was no power, the spindle was rotated for example by hand through one or more revolutions.
The state of the art affords two different basic kinds of position detector for dealing with that problem. In the first alternative configuration, coupled to the rotating shaft or spindle is a step-down transmission assembly whose output shaft rotates through a maximum of 360.degree. when the tool holder carriage passes over its entire adjustment length. The output shaft of the step-down transmission assembly is monitored by an absolute encoder which delivers an output signal which identifies the respective instantaneous angular position of the output shaft of the step-down transmission assembly and can thus serve as the coarse measurement value in respect of the instantaneous position of the carriage. Particularly when the arrangement involves long displacement travels, so that the spindle has to perform a large number of revolutions in order to cover those displacement travels, the step-down transmission assembly and the absolute encoder must satisfy extremely high levels of requirement in respect of accuracy. The play in the step-down transmission assembly must be kept so small that the uncertainty which occurs upon a reversal in the direction of rotation of the spindle is less than the angle which the absolute encoder resolves, for detecting a revolution of the spindle. It is clear that a step-down transmission assembly with absolute encoder, in order to detect a step-down ratio of for example 4000:1, requires a high level of apparatus expenditure that gives rise to correspondingly high overall costs. In addition, because of their high mass moment of inertia, such transmission assemblies are not suitable for average or high levels of acceleration and speeds of rotation.
Another alternative configuration involves designing a simple optical or magnetic detector in such a way that, whenever a marking on the rotating shaft rotates past the detector, the detector outputs an electrical signal which is fed to the electronic processing system. That arrangement is supplied with power by means of a battery so that it is independent of the main power supply of the machine tool. Although such a revolution counter involves a substantially lower level of manufacturing cost than the first alternative discussed above, it does however suffer from the disadvantage that it requires the batteries to be continuously monitored and changed in good time as required.